Auditory midbrain implant

ABSTRACT

An electrode array ( 10 ) that is implantable within the inferior colliculus of the midbrain and/or other appropriate regions of the brain of an implantee and adapted to provide electrical stimulation thereto. The electrode array ( 10 ) an elongate member ( 11 ) having a plurality of electrodes ( 12 ) mounted thereon in a longitudinal array. A delivery cannula ( 30 ) for delivering the electrode array ( 10 ) comprised of two half-pipes ( 31 ) is also described.

FIELD OF THE INVENTION

The present invention relates to an implant system that can bepositioned in the inferior colliculus of the midbrain of an implantee toprovide a hearing sensation to persons with hearing loss.

BACKGROUND OF THE INVENTION

Hearing loss can be due to many different causes. One type of hearingloss is conductive hearing loss which occurs where the normal mechanicalpathways for sound to reach the hair cells in the cochlea are impeded,for example, by damage to the ossicles. Conductive hearing loss mayoften be helped by use of conventional hearing aids, which amplify soundso that acoustic information does reach the cochlea and the hair cells.

In many people who are profoundly deaf, however, the reason for deafnessis sensorineural hearing loss. This type of hearing loss is due to theabsence of, or destruction of, the hair cells in the cochlea whichtransduce acoustic signals into nerve impulses. These people are thusunable to derive suitable benefit from conventional hearing aid systems,because there is damage to or absence of the mechanism for nerveimpulses to be generated from sound in the normal manner.

Cochlear implant systems have been developed for persons withsensorineural hearing loss which bypass the hair cells in the cochleaand directly deliver electrical stimulation to the auditory nervefibres, thereby allowing the brain to perceive a hearing sensationresembling the natural hearing sensation normally delivered to theauditory nerve. U.S. Pat. No. 4,532,930, the contents of which areincorporated herein by reference, provides a description of one type oftraditional cochlear implant system.

While cochlear implants have proven very successful in restoring hearingsensation to many people, persons with bilateral neural deafness areunable to benefit from such technology due to the missing transmissionof electrical signals to the second neuron of the auditory pathway. Suchpersons are mainly patients suffering from neurofibromatosis type II andbilateral acoustic neuromas and, less frequently, patients withcongenital missing auditory nerve or traumatic lesions of the auditorynerve.

Restoration of hearing to such persons is to date only possible withelectrical stimulation central to the lesion site, eg. the cochlearnucleus. For example, a surface electrode plate can be placed on thesurface of the cochlear nucleus in the lateral recess. While severalhundred patients have now received such implants and had somerestoration of hearing, the results in terms of performance are belowthat now achieved by cochlear implants. Postulated explanations for theresults include the distortion of the anatomy at the cochlear nucleusdue to the tumour size or previous treatment including gamma knifetherapy, unfavourable exposure with limited visibility of thestimulation site, and the unfavourable tonotopic organisation of thenucleus with irregular frequency layer organisation in relation to theplane of the electrode plate.

With the above background in mind, there is a need to provide an implantsystem that provides a hearing sensation to persons unable to derive anybenefit from conventional hearing aids and cochlear implant systems.

Further to this, with the benefits of electrical stimulation applied tospecific parts of the brain to treat disorders such as Parkinson'sDisease, Dyskinesia etc now being realised, there is a need to providean implant system that can be easily adapted to apply such treatment.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is solely forthe purpose of providing a context for the present invention. It is notto be taken as an admission that any or all of these matters form partof the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed before the priority dateof each claim of this application.

SUMMARY OF THE INVENTION

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

According to a first aspect, the present invention is an implant systemthat provides a hearing sensation to an implantee, the system comprisingan electrically stimulating electrode implantable within the inferiorcolliculus of the midbrain of the implantee to provide electricalstimulation thereto.

An advantage of placing the electrode within the inferior colliculus isthat the anatomy of this site is generally not distorted by the presenceof tumours, such as neurofibromatosis type II, or previous treatmentthereof. The inferior colliculus can also typically be exposed withrelatively good visibility and is relatively easy to identify.

The inferior colliculus is composed of several subdivisions; the centralnucleus (ICC), the dorso-medial nucleus (DM), the dorsal cortex (DC) andthe lateral nucleus (LN). Virtually every kind of pre-processed auditoryinformation from the brainstem is collected in the ICC, which from thepoint of tonotopicity, has a highly organised pattern, with layersmostly parallel to the surface of the inferior colliculus. The wholefrequency range of hearing is represented by the isofrequency planes inthe inferior colliculus, which are ordered by low frequenciesrepresented dorsolaterally and high frequencies ventromedially.

In a further aspect, the present invention is an electrode array that isinsertable in the brain of an implantee, the electrode array having aplurality of electrodes mounted thereon.

In one embodiment, the electrodes are mounted on an elongate member. Inone embodiment of both aspects, the elongate member can have betweenabout 4 and 80 electrodes, and more preferably about 20 electrodes,mounted thereon.

The electrodes can be disposed in a longitudinal array on the elongatemember. The electrodes can be equally spaced along the elongate member.The electrodes can be adapted to apply a preselected tissue stimulationto the inferior colliculus, and/or other appropriate regions of thebrain, such as the Subthalamic Nucleus (STN), the Globus Pallidus (GPi),and the Thalamus. The electrodes are preferably adapted to providemulti-channel stimulation of these regions of the brain.

Each electrode can consist of a single or several penetrating pins. Inanother embodiment, each electrode can comprise a ring. In anotherembodiment, the electrodes may comprise half rings, flat pads ormultiple pads around the circumference of the elongate body. In oneembodiment, each of the electrodes can be identical in form. In anotherembodiment, at least one of the electrodes can have a different formfrom at least one of the other electrodes. The pins can be made of asuitable electrically conducting material, such as platinum orplatinum-iridium alloy. The electrodes preferably have a surface areasufficiently large so as to not exceed charge density limits.

The spacing of the electrodes is preferably such that differentfrequency layers of the inferior colliculus can be stimulated. Eachelectrode can have a width of between about 50 and about 2000 microns,more preferably about 100 microns. The spacing between the electrodescan be between about 50 and about 2000 microns, more preferably about100 microns. The pitch of the electrodes can be between about 50 micronsand about 2000 microns, more preferably about 200 microns. In oneembodiment, the array has a length of between about 2 mm and about 6 mm,more preferably about 4 mm long.

In a preferred embodiment, the elongate member has a tip at a distal endthat assists in passage of the member into the brain or portionsthereof, such as the inferior colliculus, while causing relativelyminimal trauma to the sensitive tissues of the brain. The diameter ofthe elongate member can begin to decrease proximate the distal end tothe tip. The tip can be formed of a biocompatible material, such asstainless steel, platinum-iridium alloy or other metals, and can bemachinable to a desired geometry.

The tip can be formed from a material selected from the group comprisingsilicone, polytetrafluoroethylene (PTFE), polyurethane, other polymers,and polymer-coated substrates such as silicone-coated platinum andparylene-coated platinum. The tip can have a relatively small radius,however, a relatively larger radius tip could be used, eg. a hemispherewith the radius matching the diameter of the body of the electrode.

The elongate member can have a body formed from a suitable biocompatiblematerial. In one embodiment, the body can be formed from a materialselected from the group comprising silicone, polyimide, polyurethane,PTFE, and other polymers.

The body is preferably relatively stiff to allow the elongate member topenetrate the surface of the inferior colliculus and be inserted at adesired depth therein. The elongate body can have a diameter in therange of about 0.2 mm to about 2 mm, more preferably about 0.5 mm.

In one embodiment, the elongate member can have a stiffening elementextending at least partially therethrough. The stiffening element can benon-removably positioned in the elongate member. In another embodiment,the Stiffening element can comprise a removable stylet. The stiffeningelement can comprise a wire. The wire can be circular or non-circular incross-section. The stylet can be formed of a suitable metallic material,such as stainless steel. In another embodiment, the stiffening elementcan be formed from a bioresorbable material. The stiffening element canextend through a lumen formed in the elongate member and the lead. Thelumen preferably extends axially through the elongate member and thelead.

In one embodiment, a stiffening stylet can be used to maintain theelongate member in a configuration suitable for insertion into thedesired region. Once positioned, the stylet would preferably be removedfrom the member and the lead simply by withdrawing the stylet from thelumen. In this regard, the lead needs to be of a suitable thickness toenclose the stylet and the conducting wires connecting the electrodes tothe stimulator/receiver unit.

In an alternative embodiment, the stiffening element can have a steppedouter surface. In such an arrangement, the element can have a section ofrelatively narrower dimensions that is inserted into the elongate memberand a section of relatively wider dimensions that is normally positionedexternal of the array. In this arrangement, each section can be madefrom the same material or from a different material such as stainlesssteel, titanium, iridium etc. This will allow the relative rigidity ofeach section to the stiffening element to be altered to suit thespecific purpose. As can be appreciated, this arrangement does notrequire a lumen extending from the array through the lead as only ashort lumen is required through the array.

To remove the stylet following correct insertion of the array, anadditional holding tool can be employed to maintain the array inposition. Such a tool can be placed behind the skirt of the array whilstthe stylet is being removed. This has the benefit in that the size ofthe lead can be significantly reduced, as it no longer has toaccommodate a stiffening stylet. A relatively thinner lead also has theadvantage of being relatively more flexible. This is important when oneconsiders that the brain is in constant pulsing motion and therefore alead positioned within the brain could cause complications if it is of asize and rigidity that would cause pressure on the structures of thebrain. As it is expected that the cerebellum will need to be depressedto allow insertion of the electrode, the lead therefore passes over thetop of the cerebellum following electrode insertion. Once the cerebellumis released it will resume its shape to fill up the skull cavity and assuch the lead needs to be relatively flexible to so move with thecerebellum and not cut or bruise the cerebellum. Therefore, reducing thesize of the lead and increasing its flexibility is important in thepresent application. Further to this, by requiring only a short lumenextending into the array, the potential for creating a passage forinfection to travel from the stimulator/receiver unit to the array isreduced, reducing the risk of such infections and meningitis.

In yet another aspect, the present invention is a stiffening element fora brain stimulating electrode array wherein the stiffening element has astepped outer surface with the element having a section of relativelynarrower dimensions that is inserted into the elongate member and asection of relatively wider dimensions that is normally positionedexternal of the array.

In this aspect, the stiffening element can have the features as aredefined herein.

As mentioned, in one embodiment of any one of the aspects, the elongatemember can have a cuff or skirt positioned about the body at apredetermined distance from the distal tip. The skirt or cuff can beadapted to collapse on insertion of the elongate member through adelivery cannula but expand on exiting a distal end of the cannula. Theskirt or cuff can be formed from Dacron™.

The skirt or cuff is preferably spaced a distance of between about 6 and6.5 mm from the distal tip and is adapted to stabilise the elongatemember in the inferior colliculus and ensure the elongate member doesnot migrate further into the inferior colliculus following correctplacement.

The skirt can have a plurality of fold lines and/or ribs formed thereinso as to allow the skirt to collapse on insertion through the deliverycannula. In one embodiment, the skirt can be adapted to collapse and/orexpand in a spiral fashion.

Each electrode is preferably individually connected to at least one wirewhich is electrically insulated from other wires extending to otherelectrodes in the array. The wires preferably extend through theelongate member to at least, and preferably beyond, the proximal end ofthe elongate member. The wires can extend through a lead that extendsoutwardly from the proximal end of the elongate member.

Each elongate member can have a stiffening element extending at leastpartially therethrough. The stiffening element can be non-removablypositioned in the elongate member. In another embodiment, the stiffeningelement can comprise a removable stylet. The stiffening element cancomprise a wire. The wire can be circular or non-circular incross-section. The stylet can be formed of a suitable metallic material,such as stainless steel. In another embodiment, the stiffening elementcan be formed from a bioresorbable material. The stiffening element canextend through a lumen formed in the elongate member. Such a lumenpreferably extends axially through the elongate member.

In a further embodiment, one or more bioactive agents can be deliveredthrough the lumen of the elongate member. In one embodiment, thebioresorbable stiffening element can have one or more bioactive agentsincorporated therein. One or more ports can be formed in the elongatemember to allow the bioactive agents to elute into the site ofimplantation of the elongate member. The one or more ports can be at oradjacent the tip of the elongate member or distal thereto. The ports canalso be adapted to allow body fluids, such as cerebrospinal fluid, toenter the lumen. The entry of the body fluids can be used to cause orcontinue resorption of a bioresorbable stiffening element positionedtherein.

In one embodiment, the implant system further comprises a stimulatorthat outputs stimulation signals to the electrodes of the electrodearray according to the first aspect. Such a stimulator can also be usedin conjunction with the electrode array of the further aspect. For thepurpose of this description, the features as hereunder defined can beused in association with the invention as defined in either aspect.

The stimulator is preferably electrically connected to the elongatemember by way of an electrical lead, including the lead defined above.The lead can include the one or more wires extending from each electrodeof the array mounted on the elongate member. In one embodiment, the leadcan extend from the elongate member to the stimulator or at least thehousing thereof.

In one embodiment, the lead is continuous with no electrical connectors,at least external the housing of the stimulator, required to connect thewires extending from the electrodes to the stimulator. One advantage ofthis arrangement is that there is no requirement for the surgeonimplanting the device to make the necessary electrical connectionbetween the wires extending from the electrodes and the stimulator.

The stimulator is preferably positioned within a housing that isimplantable within the head of the implantee. The housing for thestimulator is preferably implantable within a bony well in the bonebehind the ear posterior to the mastoid.

When implantable, the housing preferably contains, in addition to thestimulator, a receiver. The receiver is preferably adapted to receivesignals from a controller. The controller is, in use, preferably mountedexternal to the body of the implantee such that the signals aretransmitted transcutaneously between the controller and the receiver.

Signals can preferably travel from the controller to the receiver andvice versa. The receiver can include a receiver coil adapted to receiveradio frequency (RF) signals from a corresponding transmitter coil wornexternally of the body. The radio frequency signals can comprisefrequency modulated (FM) signals. While described as a receiver coil,the receiver coil can preferably transmit signals to the transmittercoil which receives the signals.

The transmitter coil is preferably held in position adjacent theimplanted location of the receiver coil by way of respective attractivemagnets mounted centrally in, or at some other position relative to, thecoils.

The external controller can comprise a speech processor adapted toreceive signals output by a microphone. During use, the microphone ispreferably worn on the pinna of the implantee, however, other suitablelocations can be envisaged, such as a lapel of the implantee's clothing.The speech processor encodes the sound detected by the microphone into asequence of electrical stimuli following given algorithms, such asalgorithms developed for cochlear implant systems. The encoded sequenceis transferred to the implanted stimulator/receiver device using thetransmitter and receiver coils. The implanted stimulator/receiver devicedemodulates the FM signals and allocates the electrical pulses to theappropriate attached electrode by an algorithm which is consistent withthe chosen speech coding strategy.

The external controller preferably further comprises a power supply. Thepower supply can comprise one or more rechargeable batteries. Thetransmitter and receiver coils are used to provide power viatranscutaneous induction to the implanted stimulator/receiver device andthe electrode array.

While the implant system can rely on external componentry, in anotherembodiment, the controller, including the microphone, speech processorand power supply can also be implantable. In this embodiment, thecontroller can be contained within a hermetically sealed housing or thehousing used for the stimulator.

According to a still further aspect, the present invention is a deliverycannula for delivering an elongate member having an array of electrodesmounted thereon to a desired location in the brain of an implantee.

In one embodiment, the elongate member and/or the electrode array ofthis aspect can have the features of these devices as defined herein.

The cannula can be used in stereotactic placement of the elongate memberin the brain, such as in the inferior colliculus, the SubthalamicNucleus (STN), the Globus Pallidus (GPi), and/or the Thalamus of thebrain of the implantee.

The cannula can be comprised of two or more longitudinal portions. Inone embodiment, the cannula can be comprised of two half-pipes joined atrespective longitudinal joins. The cannula when assembled can becylindrical in form. Other forms can, however, be envisaged.

The two half-pipes are preferably held together by a holding deviceadapted to be positioned outside the skull during use of the cannula.The holding device can be a mechanical holding device, such as a ring,that removably surrounds the half-pipes. The holding device can alsocomprise a polymer sheath, such as a parylene coating, that is removablefrom the cannula. The parylene coating can be in the form of a thin filmhaving a thickness of about 3-5 microns. A wire that slits the sheathalong the joins of the half-pipes can cut the sheath. In anotherembodiment, the sheath can be cut off the cannula using a scalpel or hotknife. Still further, the holding device can comprise respective pinsthat extend the length of the cannula and join the respective edges ofthe half-pipes together.

The disassembly of the delivery cannula from around the elongate memberand/or the lead extending therefrom is advantageous when the lead isnon-removably connected from the elongate member back to the stimulatorand/or the housing therefor. The cannula can be removed from at leastaround the lead by being disassembled following insertion of theelongate member into the brain.

In a still further aspect, the present invention is a method ofproviding electrical stimulation to the brain of an implantee, themethod comprising the steps of:

-   -   (i) implanting at least one electrode array within a portion of        the brain of the implantee; and    -   (ii) delivering electrical stimulation through the array to said        portion of the brain.

In a preferred embodiment, step (i) comprises the step of implanting anelongate member having an array of electrodes as is defined herein.

In one embodiment, the method can be used to provide a hearing sensationto an implantee. In this embodiment, step (i) comprises implanting theelectrode array in the inferior colliculus of the brain of the implanteeand step (ii) comprises delivering electrical stimulation through thearray to the inferior colliculus.

In yet another embodiment, step (i) can comprise a step of implantingthe electrode array in the Subthalamic Nucleus (STN), the GlobusPallidus (GPi), and/or the Thalamus of the brain of the implantee, withstep (ii) comprising delivering electrical stimulation to these portionsof the brain.

In a further embodiment, step (i) of the method preferably comprises thesteps of:

-   -   (a) identifying the position of the portion of the brain of the        implantee that is to receive the electrode array;    -   (b) mounting a stereotactic frame to the implantee's head; and    -   (c) inserting the electrode array into said portion of the brain        using the stereotaxis.

In this embodiment, step (a) can comprise a step of identifying theinferior colliculus of the midbrain. The position of the inferiorcolliculus can preferably be localised using magnetic resonance imaging(MRI) with exact intraoperative placement adjusted by direct electricalstimulation of the inferior colliculus and recording of electricallyevoked auditory potentials. In this approach, the electrode can beplaced under local anaesthesia while verifying the optimum placement bypsychophysical measures and according to the patient's recommendations.The method therefore includes a step of confirming the correct positionof the electrodes of the array. This can be confirmed during surgery ona conscious patient by matching each electrode position with thefrequency of best response for acoustic stimuli applied to thecontralateral ear. This step will confirm that the electrodes cover thechosen acoustic frequency range. For patients with no acoustic hearingin the contralateral ear, confirmation of frequencies will be based onpatient judgement in response to stimulation of the differentelectrodes. To add electrodes, the array can be advanced into theinferior colliculus. Addition to the range of frequencies representedwill be confirmed as the electrode array is advanced.

The stereotactic implantation can be performed through a burr hole, withthe stimulator/receiver device placed subperiostally in a bony bedbehind the pinna.

Once in position, the elongate member can be fixed at the skull and thestereotactic frame removed.

There are two other presently envisaged methods for placement of theelectrode array in the inferior nucleus.

The first envisaged method involves placement during removal of anacoustic neuroma. In this case, the inferior colliculus can be reachedby a medially extended, lateral suboccipital approach, with downwardretraction of the cerebellum. The electrode can be inserted fromlaterally to antero-medially. This direction of penetration issubstantially perpendicular to the organisation of the frequency layersin the central nucleus of the inferior nucleus. The placement will beperformed at the same surgical setting, after removal of the acousticneuroma. After implantation, the electrode lead is preferably passedthrough an opening in the dura and can extend toward and preferably tothe stimulator/receiver device, such as the device defined herein.

The second envisaged method involves using a medial sub-occipital(infratentorial-supracerebellar) approach. Using this approach, removalof an acoustic neuroma in an implantee with neurofibromatosis type IIwho has already lost his/her hearing and has multiple other tumours inthe CNS is not absolutely necessary, unless the tumour is very large andhas endangered other cranial nerves (such as the facial nerve) or iscompressing the brainstem. In this approach, after downward retractionof the cerebellum, direct exposure of the inferno colliculus is possibleand the electrode can be placed under direct vision into the inferiorcolliculus.

In yet another aspect the present system is an electrode array for thetreatment of movement disorders and other neurological disorders, suchas Parkinson's disease, Dyskinesia, Tourette's Syndrome, EssentialTremor and Epilepsy, to name a few.

In this regard, the electrode array preferably has a plurality ofelectrodes capable of being inserted into appropriate regions of thebrain, such as the Subthalamic Nucleus (STN), the Globus Pallidus (GPi),and the Thalamus for multi-channel stimulation of these regions. Byproviding an increased number of electrodes inserted into these regionswith the present invention, treatment can be optimised to suit thespecific disorder being addressed.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, preferred embodiments of the invention are nowdescribed with reference to the accompanying drawings, in which:

FIG. 1 is a side elevation view of one embodiment of an electrode arrayaccording to the present invention;

FIG. 2 is a plan view of the electrode of FIG. 1 but also depicting thelead extending back to the stimulator/receiver of the implant system;

FIG. 3 a is a perspective view and FIG. 3 b an end elevation view of oneembodiment of a delivery cannula for use in the positioning of anelectrode, such as that depicted in FIG. 1;

FIG. 4 is an end elevation view of an alternative embodiment of adelivery cannula for use in the positioning of an electrode;

FIG. 5 is a perspective view of a stylet for use in the array accordingto the present invention;

FIG. 6 is a simplified view of another arrangement for mounting a styletin an array according to the present invention;

FIG. 7 is a simplified view of the Inferior colliculus (IC) and thecentral nucleus (ICC) of the brain of an implantee having received thearray according to the present invention;

FIG. 8 is a simplified view of the brain and cerebellum of an implantee.

PREFERRED MODE OF CARRYING OUT THE INVENTION

One embodiment of an electrode array that is implantable within theinferior colliculus of the midbrain of an implantee and adapted toprovide electrical stimulation thereto is depicted generally as 10 inFIGS. 1 and 2.

While described as implantable in the inferior colliculus, it is to beappreciated that the electrode array and the system in general asdescribed below can, with necessary appropriate modification, also beused in the treatment of movement disorders and other neurologicaldisorders, such as Parkinson's disease, Dyskinesia, Tourette's Syndrome,Essential Tremor and Epilepsy, to name a few. In this regard, it is tobe understood that the electrode array is also suitable for insertioninto other appropriate regions of the brain, such as the SubthalamicNucleus (STN), the Globus Pallidus (GPi), and the Thalamus to providemulti-channel stimulation of these regions. By providing an increasednumber of electrodes inserted into these regions with the presentinvention, treatment can be optimised to suit the specific disorderbeing addressed.

As determined by the present inventors, an advantage of placing theelectrode array within the inferior colliculus is that the anatomy ofthis site is generally not distorted by the presence of tumours, such asa neurofibromatosis type 11 tumour, or previous treatment thereof. Theinferior colliculus can also be exposed with relatively good visibilityand is relatively easy to identify.

Implantation within the inferior colliculus is envisaged as desirable asvirtually every kind of pre-processed auditory information from thebrainstem is collected in the inferior colliculus, which from the pointof tonotopicity, has a highly organised pattern, with layers mostlyparallel to the surface of the inferior colliculus. The whole frequencyrange of hearing is represented by the isofrequency planes in theinferior colliculus, which are ordered by low frequencies representeddorsolaterally and high frequencies ventromedially.

In the depicted embodiment, the electrode array has an elongate member11 having a plurality of electrodes 12 mounted thereon. In thisembodiment, the elongate member has twenty electrodes 12 disposed in alongitudinal array on the elongate member 11. As depicted, theelectrodes 12 can be equally spaced along the elongate member 11 andadapted to apply a preselected tissue stimulation to the inferiorcolliculus.

In this embodiment, each electrode 12 comprises a platinum orplatinum-iridium alloy ring. In another embodiment, the electrodes maycomprise half rings, flat pads, or multiple pins or pads around thecircumference of the elongate body 11.

The spacing of the electrodes is preferably such that differentfrequency layers of the inferior colliculus can be stimulated. In thedepicted embodiment, each electrode has a width of 100 microns, with thespacing between the electrodes being 100 microns. The total length ofthe depicted elongate member 11 from the outer (as depicted, the left)edge of ring one 13 to the outer (as depicted, the right) edge of ringtwenty 15 is 4.1 mm.

The depicted elongate member 11 has a tip 16 at a distal end thatassists in passage of the member 11 into the brain or portions thereof,such as the inferior colliculus, while causing relatively minimal traumato the sensitive tissues of the brain. As depicted, the diameter of theelongate member can begin to decrease proximate the distal end to thetip. The commencement of the tapering of the tip 16 is spaced 0.2 mmfrom the outer edge of ring twenty 15 and extends for a length of 1 mm.

The tip can be formed of a biocompatible material, such as stainlesssteel, platinum-iridium alloy or other metals and be machinable to adesired geometry. The tip can be formed from a material selected fromthe group comprising silicone, polytetrafluoroethylene (PTFE),polyurethane, other polymers, and polymer-coated substrates such assilicone-coated platinum and parylene-coated platinum.

The elongate member 11 can have a body formed from a suitablebiocompatible material. In one embodiment, the member 11 can be formedfrom a material selected from the group comprising silicone, polyimide,polyurethane, PTFE, and other polymers.

The member 11 is preferably sufficiently relatively stiff to allow theelongate member 11 to penetrate the surface of the inferior colliculusand be inserted at a desired depth therein. The depicted elongate memberhas a diameter of 0.5 mm in the region apart from the tip 16.

The member 11 can have a stiffening element extending at least partiallytherethrough. The stiffening element can be non-removably positioned inthe elongate member 11. In another embodiment, the stiffening elementcan comprise a removable stylet. The stiffening element can comprise awire. The wire can be circular or non-circular in cross-section. Thestylet can be formed of a suitable metallic material, such as stainlesssteel. In another embodiment, the stiffening element can be formed froma bioresorbable material. The stiffening element can extend through alumen formed in the elongate member and the lead 18. The lumenpreferably extends axially through the elongate member 11 and lead 18.

In the embodiment shown in FIG. 2, a stiffening stylet can be used tomaintain the elongate member in a configuration suitable for insertioninto the desired region. Once positioned, the stylet is removed from themember 11 and the lead 18 simply by withdrawing the stylet from thelumen. In this regard, the lead 18 is of a suitable thickness to enclosethe stylet and the conducting wires connecting the electrodes to thestimulator/receiver unit 21.

In an alternative embodiment, the stiffening element can be a steppedstylet 60 as shown in FIG. 5. In such an arrangement, the narrowersection shown as A is inserted into the elongate member 11 forinsertion, with the wider section B being external of the array 10 as isshown in FIG. 6. Each of the sections A and B can be made from the samematerial, for example stainless steel, titanium, iridium or tungsten, oreach section could be made from a different material. This may allow forone section, eg section A, being more/less rigid than the other section,eg section B. As can be appreciated, this arrangement does not require alumen extending from the array 10 through the lead 18 as only a shortlumen is required through the array 10.

To remove the stylet 60 following correct insertion of the array 10, anadditional holding tool 70 can be employed to maintain the array 10 inposition. Such a tool can be placed behind the skirt 17 of the array 10whilst the stylet 60 is being removed. This arrangement has the benefitin that the size of the lead 18 can be significantly reduced and thusthe rigidity, as it no longer has to accommodate a stiffening stylet.This is important when one considers that the brain is in constantpulsing motion and therefore a lead positioned within the brain couldcause complications if it is of a size and rigidity that would causepressure on the structures of the brain. Therefore, reducing the size ofthe lead and increasing its flexibility is important in the presentapplication. Further to this, by requiring only a short lumen extendinginto the array 10, the potential for creating a passage for infection totravel from the stimulator/receiver unit to the array 10 is reduced,reducing the risk of such infections and meningitis. Also, having ashort stylet in the shorter lumen will reduce the friction when thestylet is withdrawn. This therefore allows for a gentler surgicalprocedure then that which would require removal of a stylet from alonger lumen.

As depicted, the elongate member 11 can have a skirt 17 positioned aboutthe member at a predetermined distance from the tip 16. The skirt in thedepicted embodiment has a radius of 4 mm and is 0.2 mm thick. The skirt17 can be adapted to collapse on insertion of the elongate member 11through a delivery cannula 30, as described below, but expand on exitinga distal end of the cannula 30. The skirt can be formed from Dacron™.The depicted skirt 17 is spaced a distance of 1.2 mm from the outer edgeof ring one 13 and is adapted to stabilise the elongate member 11 in theinferior colliculus and ensure the elongate member 11 does not migratefurther into the inferior colliculus following correct placement. Asshown in FIG. 7, tie spacing from the outer edge of ring one 13 to theskirt caters for the distance between the Inferior colliculus (IC) andthe central nucleus (ICC) where every kind of preprocessed auditoryinformation from the brainstem is collected in a highly organised andknown pattern as shown, which can be stimulated by the electrodes.

The skirt 17 can have a plurality of fold lines and/or ribs formedtherein so as to allow the skirt to collapse on insertion through thedelivery cannula. In one embodiment, the skirt 17 can be adapted tocollapse and/or expand in a spiral fashion.

Each electrode 12 is individually connected to at least one wire whichis electrically insulated from other wires extending to other electrodes12 in the array. The wires (not visible) extend through the elongatemember 11 and out the proximal end of the elongate member 11 throughlead 18.

The lead 18 can extend to a stimulator/receiver 21 that outputsstimulation signals to the electrodes 12 of the elongate member 11. Thedepicted lead 18 from the stimulator/receiver 21 to the skirt 17 has alength of 180 mm and a diameter of 1.1 mm or less.

In the depicted embodiment, the lead 18 is continuous with no electricalconnectors, at least external the housing of the stimulator/receiver 21,required to connect the wires extending from the electrodes 12 to thestimulator/receiver 21. One advantage of this arrangement is that thereis no requirement for the surgeon implanting the device to make thenecessary electrical connection between the wires extending from theelectrodes 12 and the stimulator/receiver 21 during the surgery.

In an alternative embodiment, the lead 18 could be removably connectableto the stimulator/receiver 21 through a connector system. Such aconnector system provides a surgeon the ability to explant thestimulator/receiver 21 without needing to remove and reimplant theelectrode array 10.

The stimulator/receiver 21 is positioned within a hermetically sealedhousing 22 that is implantable within a bony well in the bone behind theear posterior to the mastoid of the implantee.

The stimulator/receiver is also adapted to receive signals from acontroller. The controller is, in use, preferably mounted external tothe body of the implantee such that the signals are transmittedtranscutaneously through the implantee.

Signals can preferably travel from the controller to thestimulator/receiver 21 and vice versa. The stimulator/receiver caninclude a receiver coil adapted to receive radio frequency (RF) signalsfrom a corresponding transmitter coil worn externally of the body. Theradio frequency signals can comprise frequency modulated (FM) signals.

The transmitter coil is preferably held in position adjacent theimplanted location of the receiver coil by way of respective attractivemagnets mounted centrally in, or at some other position relative to, thecoils.

The external controller can comprise a speech processor adapted toreceive signals output by a microphone. During use, the microphone ispreferably worn on the pinna of the implantee, however, other suitablelocations can be envisaged, such as a lapel of the implantee's clothing.The speech processor encodes the sound detected by the microphone into asequence of electrical stimuli following given algorithms, such asalgorithms developed for cochlear implant systems. The encoded sequenceis transferred to the implanted stimulator/receiver 21 using thetransmitter and receiver coils. The implanted stimulator/receiver 21demodulates the FM signals and allocates the electrical pulses to theappropriate attached electrode 12 by an algorithm which is consistentwith the chosen speech coding strategy.

The external controller can further comprise a power supply. The powersupply can comprise one or more rechargeable batteries. The transmitterand receiver coils are used to provide power via transcutaneousinduction to the implanted stimulator/receiver 21 and the electrodearray 10.

While the implant system can rely on external componentry, in anotherembodiment, the controller, including the microphone, speech processorand power supply can also be implantable. In this embodiment, thecontroller can be contained within a hermetically sealed housing or thehousing 22 used for the stimulator/receiver 21.

Where present, one or more bioactive agents can be delivered through alumen of the elongate member 11, including a lumen adapted to receive astiffening element. In one embodiment, a bioresorbable stiffeningelement having one or more bioactive agents incorporated therein can bepositioned in the lumen. One or more ports can be formed in the elongatemember 11 to allow the bioactive agents to elute into the site ofimplantation of the elongate member 11. The one or more ports can be ator adjacent the tip 16 of the elongate member 11 or distal thereto. Theports can also be adapted to allow body fluids, such as cerebrospinalfluid, to enter the lumen. The entry of the body fluids can be used tocause or continue resorption of a bioresorbable stiffening elementpositioned therein.

In a further embodiment, at least a portion of an outer surface of theelongate member can have a coating of a lubricious material. In oneembodiment, a substantial portion or the entire outer surface of theelongate member can have a coating of the lubricious material.

In this embodiment, the lubricious material can be selected from thegroup comprising polyacrylic acid (PAA), polyvinyl alcohol (PVA),polylactic acid (PLA) and polyglycolic acid (PGA). It is envisaged thatother similar materials could also be used.

One embodiment of a delivery cannula for delivering an elongate member11 having an array of electrodes 12 mounted thereon to a desiredlocation in the brain of an implantee is depicted generally as 30 inFIGS. 3 a and 3 b.

The cannula 30 can be used in stereotactic placement of the elongatemember 11 in the brain, such as in the inferior colliculus of themidbrain of the implantee. As depicted in FIG. 3 a the cannula 30extends through the surface 41 of the skull 42 and is used to ensureaccurate delivery of the elongate member 11 into the inferiorcolliculus.

The depicted cannula 30 is comprised of two longitudinal half-pipeportions 31, joined at respective longitudinal joins 32. The cannula 30,when assembled, is cylindrical in form. Other forms can, however, beenvisaged.

The two half-pipes 31 are depicted held together in FIG. 3 a by a ring34 that removably surrounds the half-pipes 31. On removal of the ring34, the half pipes 33 can disengage along the joins 32 and be removedfrom around the lead 18.

In another embodiment, the half-pipes 31 can be held together by apolymer sheath, such as a parylene coating, that is removable from thecannula 30. The parylene coating can be in the form of a thin filmhaving a thickness of about 3-5 microns. A wire that slits the sheathalong the joins 32 of the half-pipes 31 can cut the sheath. In anotherembodiment, the sheath can be cut off the cannula using a scalpel or hotknife. Still further, the half-pipes 31 can be held together byrespective pins 35 that extend the length of the cannula 30 and join therespective edges of the half-pipes 31 together, as is depicted in FIG.4.

The disassembly of the delivery cannula 30 from around the elongatemember 11 and/or the lead 18 extending therefrom is advantageous whenthe lead 18 is non-removably connected from the elongate member 11 backto the stimulator/receiver housing 22. The cannula 30 can be removedfrom at least around the lead 18 by the surgeon by being disassembledfollowing insertion of the elongate member 11 into the brain.

In use, the elongate member 11 is implanted within the inferiorcolliculus of the implantee. Electrical stimulation is then delivered tothe inferior colliculus by the electrodes 12.

The step of implanting the elongate member 11 can comprise the steps of:

-   -   (a) identifying the position of the inferior colliculus in the        implantee;    -   (b) mounting a stereotactic frame to the implantee's head; and    -   (c) inserting the elongate member 11 into the inferior        colliculus of the midbrain using the stereotaxis.

The position of the inferior colliculus can be localised using magneticresonance imaging (MRI) with exact intraoperative placement adjusted bydirect electrical stimulation of the inferior colliculus and recordingof electrically evoked auditory potentials. In this approach, theelongate member 11 can be placed under local anaesthesia while verifyingthe optimum placement by psychophysical measures and according to thepatient's recommendations. The method therefore includes a step ofconfirming the correct position of the member 11. This can be confirmedduring surgery on a conscious patient by matching the position of eachelectrode 12 with the frequency of best response for acoustic stimuliapplied to the contralateral ear. This step will confirm that theelectrodes 12 cover the chosen acoustic frequency range. For patientswith no acoustic hearing in the contralateral ear, confirmation offrequencies will be based on patient judgement in response tostimulation of the different electrodes 12. To add electrodes 12, theelongate member 11 can be advanced into the inferior colliculus.Addition to the range of frequencies represented will be confirmed asthe elongate member 11 is advanced.

The stereotactic implantation can be performed through a burr hole, withthe stimulator/receiver 21 placed subperiostally in a bony bed behindthe pinna.

Once in position, the elongate member 11 can be fixed at the skull 42and the stereotactic frame removed.

There are two other envisaged methods for placement of the electrodearray on the elongate member 11. The first involves placement duringremoval of an acoustic neuroma. The inferior colliculus can be reachedby a medially extended, lateral suboccipital approach, with downwardretraction of the cerebellum 51 of an implantee 50 as shown by arrow Cin FIG. 8. The elongate member 11 can be inserted from laterally toantero-medially. This direction of penetration is substantiallyperpendicular to the organisation of the frequency layers in the centralnucleus of the inferior nucleus as shown by FIG. 7. The placement willpreferably be performed at the same surgical setting, after removal ofthe acoustic neuroma. After implantation, the electrode lead 18 ispreferably passed through an opening in the dura and can extend towardand preferably to the stimulator/receiver 21.

The second approach involves a medial sub-occipital(infratentorial-supracerebellar) approach. Using this approach, removalof an acoustic neuroma in an implantee with neurofibromatosis type 11who has already lost his/her hearing and has multiple other tumours inthe CNS is not absolutely necessary, unless the tumour is very large andhas endangered other cranial nerves (such as the facial nerve) or iscompressing the brainstem. In this approach after downward retraction ofthe cerebellum 51, direct exposure of the inferior colliculus ispossible and the elongate member 11 can be placed under direct visioninto the inferior colliculus.

The present invention provides an alternative system for providinghearing sensation to persons unable to derive any or sufficient benefitfrom traditional hearing aids or cochlear implants systems.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. A brain stimulating electrode array comprising an elongate memberhaving a plurality of spaced electrodes thereon, the elongate memberbeing insertable into an inferior colliculi of an implantee and whereinthe electrodes are spaced along the member to apply preselected tissuestimulation to different frequency layers of the inferior colliculi. 2.The brain stimulating electrode array of claim 1 wherein the elongatemember has between about 4 and 80 electrodes mounted thereon.
 3. Thebrain stimulating electrode of claim 2 wherein the elongate member hasabout 20 electrodes mounted thereon.
 4. The brain stimulating electrodearray of claim 1 wherein the electrodes are disposed in a longitudinalarray on the elongate member.
 5. The brain stimulating electrode arrayof claim 4 wherein the electrodes are equally spaced along the elongatemember.
 6. The brain stimulating electrode array of claim 1 wherein eachelectrode comprises at least one single penetrating pin.
 7. The brainstimulating electrode array of claim 1 wherein each electrode isselected from a group comprising a ring, a half ring, a flat pad, and amultiple number of pains or pads around the circumference of theelongate body.
 8. The brain stimulating electrode array of claim 7wherein each of the electrodes are identical in form.
 9. The brainstimulating electrode array of claim 7 wherein at least one of theelectrodes has a different form from at least one of the otherelectrodes.
 10. The brain stimulating electrode array of claim 1 whereineach electrode has a width of between about 50 and about 2000 microns.11. The brain stimulating electrode array of claim 10 wherein eachelectrode has a width of about 100 microns.
 12. The brain stimulatingelectrode array of claim 1 wherein the spacing between the electrodes isbetween about 50 and about 2000 microns.
 13. The brain stimulatingelectrode array of claim 12 wherein the spacing between the electrodesis about 100 microns.
 14. The brain stimulating electrode array of claim1 wherein the pitch of the electrodes is between about 50 microns andabout 2000 microns.
 15. The brain stimulating electrode array of claim14 wherein the pitch of the electrodes is about 200 microns.
 16. Thebrain stimulating electrode array of claim 1 wherein the array has alength of between about 2 mm and about 6 mm.
 17. The brain stimulatingelectrode array of claim 16 wherein the array has a length of about 4 mmlong.
 18. The brain stimulating electrode array of claim 1 wherein theelongate member has a tip at a distal end that assists in passage of themember into the brain while causing relatively minimal trauma to thesensitive tissues of the brain.
 19. The brain stimulating electrodearray of claim 18 wherein the diameter of the elongate member begins todecrease proximate the distal end to the tip.
 20. The brain stimulatingelectrode array of claim 19 wherein the tip is formed of a biocompatiblematerial.
 21. The brain stimulating electrode array of claim 1 whereinthe elongate member has a body formed from a relatively stiffbiocompatible material to allow the elongate member to penetrate thesurface of the brain and be inserted at a desired depth therein.
 22. Thebrain stimulating electrode array of claim 21 wherein the elongate bodyhas a diameter of about 0.2 mm to about 2 mm.
 23. The brain stimulatingelectrode array of claim 22 wherein the elongate body has a diameter ofabout 0.5 mm.
 24. The brain stimulating electrode array of claim 18wherein the elongate member has a cuff or skirt positioned about thebody at a predetermined distance from the distal tip.
 25. The brainstimulating electrode array of claim 24 wherein the skirt or cuff isadapted to collapse on insertion of the elongate member through adelivery cannula but expand on exiting a distal end of the cannula. 26.The brain stimulating electrode array of claim 25 wherein the skirt orcuff is spaced a distance of between about 6 and 6.5 mm from the distaltip and is adapted to stabilize the elongate member in a portion of thebrain and ensure the elongate member does not migrate further into saidportion following correct placement.
 27. The brain stimulating electrodearray of claim 26 wherein the skirt has a plurality of fold lines and/orribs formed therein so as to allow the skirt to collapse on insertionthrough the delivery cannula.
 28. The brain stimulating electrode arrayof claim 27 wherein the skirt is adapted to collapse and/or expand in aspiral fashion.
 29. The brain stimulating electrode array of claim 1wherein the elongate member has a stiffening element extending at leastpartially therethrough.
 30. An implant system that provides a hearingsensation to an implantee, the system comprising an electricallystimulating electrode array as defined in claim
 1. 31. The implantsystem of claim 30 wherein the system further comprises a stimulatorthat outputs stimulation signals to the electrodes of the electrodearray.
 32. The implant system of claim 31 wherein the stimulator iselectrically connected to the elongate member by way of an electricallead having one or more wires extending from each electrode of thearray.
 33. The implant system of claim 32 wherein the stimulator ispositioned within a housing that is implantable within the head of theimplantee.
 34. The implant system of claim 33 wherein the housingfurther contains a receiver that receives signals from a controllermounted external to the body of the implantee such that the signals aretransmitted transcutaneously between the controller and the receiver.35. The implant system of claim 34 wherein the receiver includes areceiver coil adapted to receive radio frequency (RF) signals form acorresponding transmitter coil worn externally of the body.
 36. Theimplant system of claim 35 wherein the external controller comprises aspeech processor that receives signals output by a microphone, encodesthe sound detected by the microphone into a sequence of electricalstimuli following given algorithms, and then transfers the encodedsequence to the implanted device using the transmitter and receivercoils.
 37. The implant system of claim 36 wherein the externalcontroller further comprises a power supply, with the transmitter andreceiver coils providing power via transcutaneous induction to theimplanted device and the electrode array.
 38. The implant system ofclaim 32 wherein the elongate member has a stiffening element extendingat least partially therethrough.
 39. The implant system of claim 38wherein the stiffening element comprises a removable stylet extendingthrough a lumen formed in the elongate member and the lead.
 40. Theimplant system of claim 38 wherein the stiffening element has a steppedouter surface with the element having a section of relatively narrowerdimensions that is inserted into elongate member and a section ofrelatively wider dimensions that is normally positioned external of thearray.
 41. A stiffening element when used with a brain stimulatingelectrode array as defined in claim 1 wherein the stiffening element hasa stepped outer surface with the element having a section of relativelynarrower dimensions that is inserted into the electrode array and asection of relatively wider dimensions that is normally positionedexternal of the array.
 42. A delivery cannula for delivering anelectrode array as defined in claim
 1. 43. The delivery cannula of claim42 wherein the cannula is comprised of two or more longitudinal portionsjoined at respective longitudinal joins.
 44. The delivery cannula ofclaim 43 wherein the two more longitudinal portions comprise twohalf-pipes, with the cannula when assembled being cylindrical in form.45. The delivery cannula of claim 44 wherein the two half-pipes are heldtogether by a holding device adapted to be positioned outside the skullduring use of the cannula.
 46. The delivery cannula of claim 45 whereinthe holding device is a ring that removably surrounds the half-pipes.47. A method of providing a hearing sensation to an implantee, themethod comprising the steps of: implanting at least one electrode arrayas defined in claim 1 within the inferior colliculus of the implantee;and delivering electrical stimulation to the inferior colliculus. 48.The method of providing a hearing sensation to an implantee of claim 47wherein step (i) of the method further comprises the steps of:identifying the position of the inferior colliculus in the implantee;mounting a sterotactic frame to the implantee's head; and inserting theelectrode array into the inferior colliculus of the midbrain using thestereotaxis.
 49. The method of providing a hearing sensation to animplantee of claim 48 wherein the position of the inferior colliculus islocalized using magnetic resonance imaging (MRI) with exactintraoperative placement adjusted by direct electrical stimulation ofthe inferior colliculus and recording of electrically evoked auditorypotentials.
 50. The method of providing a hearing sensation to animplantee of claim 49 wherein the stereotactic implantation is performedthrough a burr hole, with a stimulator/receiver device placedsubperiostally in a bony bed behind the pinna.
 51. A brain stimulatingelectrode array comprising an elongate member having a plurality ofspaced electrodes thereon, the member having a lumen extending at leastpartially therethrough for a length, and a stiffening element removablypositionable in at least a portion of the lumen. 52-53. (Cancelled)